1. Field of the Invention
The present invention generally relates to the field of telecommunication networks and, in particular, to the field of optical access networks. More in particular, the present invention relates to a method and apparatus for transmitting data packets, in particular Ethernet packets, with different precedence through a passive optical network, for instance a GPON. Finally, the present invention relates to a telecommunication system.
2. Description of the Related Art
As it is known, an access network is a telecommunication network adapted to connect one or more users to a packet switched network.
In particular, an optical access network is an access network comprising optical fibers, active optical components (amplifiers, regenerators, switches and the like) and/or passive optical components (couplers, splitters, and the like). Different types of optical access networks are known in the art: aggregated point-to-point networks with a single channel for optical fiber, aggregated multi-channel point-to-point networks, spatially distributed WDM network, etc.
A passive optical network is a particularly advantageous type of optical access network. Such a passive optical network mainly comprises passive optical components, arranged according to a point-to-multipoint architecture, thus allowing to connect a plurality of users to a single node of a packet-switched network.
Typically, a passive optical network comprises one or more optical trees, each optical tree comprising single-mode optical fiber spans which are connected by splitters and couplers.
The roots of the optical trees are connected to an optical line termination, which acts as a network-side interface for the passive optical network. The optical line termination is typically connected to a node of the packet-switched network.
Each leaf of each optical tree is connected to a respective optical network unit, which acts as a user-side interface for the passive optical network. An optical line termination may be connected through a passive optical network to a number of optical network units. Each optical network unit is connected to a number of users.
In a passive optical network, data packets may be transmitted either from the packet-switched network to a user (downstream), or from a user to the packet-switched network (upstream). Downstream data packets may be addressed to a single user (unicast), to more than one user (multicast), or to all the users connected to an optical line termination (broadcast).
Typically, a downstream data packet is received from the packet-switched network at the optical line termination, which encapsulates it in an encapsulation frame, as it will be described in greater detail herein after. The optical line termination then transmits the encapsulation frame across an appropriate optical tree of the passive optical network. The encapsulation frame is thus received by all the optical network units connected to said optical tree. Each optical network unit determines whether the received encapsulation frame is addressed to one of its users, as it will be explained in further detail herein after. In the affirmative, the data packet is extracted from the encapsulation frame, and it is forwarded towards the target user.
Similarly, each optical network unit receives one or more upstream data packets from its user(s). Each optical network unit encapsulates such data packets in respective encapsulation frames, as it will be explained in greater detail herein after, and it transmits such encapsulation frames across the passive optical network. Each optical network unit is associated to a respective transmission period, so that encapsulation frames from different optical network units are TDMA multiplexed. Upon reception of such multiplexed encapsulation frames, the optical line termination extracts all the data packets from the respective encapsulation frames, and it forwards them to the packet-switched network.
Different types of optical passive networks are known in the art, such as ATM Passive Optical Networks (APON), Broadband Passive Optical Networks (BPON), Ethernet Passive Optical Networks (EPON) and Gigabit Passive Optical Networks (GPON). Each type of passive optical network is adapted to support transmission of different types of packet-switched traffic, at different speeds.
In particular, the GPON passive optical network (or briefly GPON network, in the following description), which is defined by the Recommendation Series ITU-T G984.x, allows to transport voice, data and video traffic coming from/addressed to any type of packet-switched network (Ethernet, MPLS, IP, etc.), at speeds up to 2.5 Gbit/s.
As described by Recommendation ITU-T G.984.3, a GPON network supports both:
ATM encapsulation; and
GEM encapsulation (“GPON Encapsulation Mode”).
In particular, GEM encapsulation provides for inserting an upstream or downstream data packet, independently from its format and size, into the payload of a GEM frame. Such a GEM frame comprises, in addition to the payload, a header. Such a header comprises several fields, including a Port Identifier (or Port-ID). The Port-ID field, whose size is typically 12 bits, may comprise information about destination (downstream) or source (upstream) of a GEM frame. Each optical network unit may be associated to one or more values of the Port-ID field.
Thus, for instance, when an optical line termination encapsulates a downstream data packet in a GEM frame, it inserts into the Port-ID field one of the values associated the optical network unit to which the destination user of such a downstream data packet is connected.
When an optical network unit receives such a GEM frame, it checks whether the Port-ID field comprises one of its own associated values. In the affirmative, the optical network unit extracts the data packet from the GEM frame and forwards it to the user; otherwise, the optical network unit discards such a received GEM frame.
As already mentioned, a GPON network is capable of transporting different types of data packets. In particular, a GPON is capable of transporting Ethernet packets.
As it is known, an Ethernet packet comprises data and a header with several fields. Among these fields, for instance, there is a Destination Address field, which typically comprises the MAC address of the user which the Ethernet packet is addressed to.
Optionally, an Ethernet packet may be “VLAN tagged”, as defined by the Recommendation IEEE 802.1Q, Chapter 9. A VLAN tagged Ethernet packet comprises, in its Ethernet header, an additional VLAN header, which comprises a first field which is called VLAN Identifier and a second field which is called priority.
More particularly, the VLAN Identifier field, whose length is 12 bits, identifies the Virtual LAN to which the Ethernet packet is associated. Ethernet packets addressed to a same user may be tagged with the same VLAN Identifier value (VLAN tagging per user); otherwise, Ethernet packets providing the same service may be tagged with the same VLAN Identifier value (VLAN tagging per service).
The priority field, whose length is typically 3 bits, comprises the priority value of the Ethernet packet, which ranges from 0 to 7 (0 being the lowest priority, 7 being the highest priority).
The VLAN header is then 15 bit-long, and it allows a service provider to provide services with different Quality of Service through an Ethernet network. In other words, the VLAN header is indicative of the Ethernet packet precedence, i.e. of the precedence according to which the Ethernet packet has to be handled by a node.